drm/bridge: adv7511: Reset registers on hotplug

The bridge loses its hw state when the cable is unplugged. If we detect
this case in the hpd handler, reset its state.

Reported-by: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703165648.120401-1-seanpaul@chromium.org